Today's internal combustion engines all suffer from low efficiency. Various experiments have been made to increase the efficiency. For instance, lighter materials have been developed and the friction has been reduced by taking various measures. This has resulted in a certain increase in efficiency. Since it has been considered impossible to significantly improve the efficiency, the development has been concentrated on an increase of the power in relation to the cylinder volume.
Decisive factors, however, are the ratio of compression and how well the thermal energy is recovered before the combustion chamber opens to the exhaust side. However, the ratio of compression is limited by the fact that the pressure that is being built up in the combustion chamber of the cylinder during the compression phase is not allowed to be too high, since the mixture of air and fuel that is compressed in advance then ignites spontaneously. The leaner the mixture, i.e. the larger the amount of air in relation to the amount of fuel, the higher compression can be used, but instead one reaches a point where the mixture will be difficult to ignite. For the engine to smoothly operate in a wide load range, a compromise must be chosen, which at present means that the efficiency is slightly more than 35%.
With respect to recovering the thermal energy, a constant problem is that the entire combustion chamber is ventilated after each combustion. It is impossible to completely burn, during all load cases, all fuel in the entire combustion chamber. Therefore, a greater or smaller part of unburnt fuel will be released together with the exhaust gases. This problem is particularly evident in two-stroke engines. Furthermore, part of the thermal energy is used to eject the exhaust gases through the exhaust.
In particular for two-stroke engines, merely a small amount is converted into kinetic energy, while the major part escapes together with the exhaust gases. This depends on the combustion occurring merely during about a quarter of a crankshaft revolution. Subsequently use is made of the excess pressure produced by the combustion in the combustion chamber in order to eject the exhaust gases and simultaneously take in a new volume of fuel and air. The new volume is compressed during the latter part of the revolution so as to be ignited at, or in practice a certain angle before, 360/0 degrees. The best combustion which is possible to achieve by this mode of operation is achieved at fairly high speeds/high load. Particularly during idling and at lower speeds, a very incomplete combustion takes place and, thus, the thermal energy is utilised to an unsatisfactory extent.
Experiments have been made to provide an internal combustion engine which better utilises the thermal energy by a more complete combustion, as disclosed in U.S. Pat. No. 3,113,561. This patent specification discloses a stratified charge two-cycle engine, in which the amount of fuel-air mixture supplied to the combustion chamber is controlled. What one wants to achieve is to retain, during low-power operation, part of the exhaust gases in the combustion chamber to make them participate in a plurality of combustion cycles and be burnt more completely. At the same time, only part of the combustion chamber is filled with fresh fuel-air mixture, and one tries to prevent the exhaust gases and the fuel-air mixture from mixing with each other. In order to accomplish this, each cylinder has been provided with two intakes, a first intake through which merely air is supplied and a second intake through which a mixture of air and fuel is supplied. To each intake are connected a compressor and a spill valve such that the charging pressure can be controlled individually. The first intake is arranged in the lower part of the cylinder wall, practically just opposite an exhaust gas outlet, and the second intake is arranged in the top of the cylinder. The second intake is opened/closed by means of a valve. In addition to the conventional combustion chamber, two extra chambers are arranged in the upper part of the cylinder, after the valve. In case of a low power output, the first spill valve is completely open and the second is almost completely open. The mixture fills only part of the extra chambers, in which ignition takes place. The remaining part of the combustion chamber is filled with exhaust gases, which participate in the combustion. If a higher power output is required, first the second spill valve is progressively closed, then the first spill valve, which means that a smaller and smaller part of the combustion chamber is occupied by exhaust gases. Obvious drawbacks of this solution is the doubling of the components on the intake side. Moreover, the air supplied via the first intake will entrain the exhaust gases which are closest to the piston and which are least burnt through the exhaust which is located just opposite, which in a wide intermediate power range considerably reduces the advantages of the construction.